Today I noticed in my salesforce org when i change Account Owner, All the related contact owners are changing automatically. I tried to search in whole org to find if any process is running behind to change but i found none.
I even tried it in another developer edition it is behaving the same.
Is this the expected behavior? Or did something recently changed in salesforce.
Yes, this is expected behavior.
Changing the Account owner automatically changes the Contact owner.
Same principle applies to changing the Case owner, if case has tasks then Task's assigned to will get changed.
If you dont want to change the contact owner based on account ownership then you need to write a trigger to handle this.
With little more search i was able to find supporting docuemntation(https://help.salesforce.com/articleView?id=admin_transfer.htm&type=5):
Transfer of Associated Items
When you change record ownership, some associated items that are owned by the current record owner are also transferred to the new owner.
Record Associated items that are also transferred
Accounts Contacts (on business accounts only), attachments, notes, open activities, open opportunities owned by the current account owner, and optionally, closed opportunities and open opportunities owned by other users. Leads Open activities. When transferring leads to a queue, open activities are not transferred.
